Natural Gas Liquefaction by a High Pressure Expansion Process

ABSTRACT

A method and system for liquefying a methane-rich high-pressure feed gas stream using a system having first and second heat exchanger zones and a compressed refrigerant stream. The compressed refrigerant stream is cooled and directed to the second heat exchanger zone to additionally cool it below ambient temperature. It is then expanded and passed through the first heat exchanger zone such that it has a temperature that is cooler, by at least 5° F., than the highest fluid temperature within the first heat exchanger zone. The feed gas stream is passed through the first heat exchanger zone to cool at least part of it by indirect heat exchange with the refrigerant stream, thereby forming a liquefied gas stream. At least a portion of the first warm refrigerant stream is directed to the second heat exchanger zone to cool the refrigerant stream, which is compressed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. ProvisionalApplication No. 62/565,725 filed Sep. 29, 2017 entitled “Natural GasLiquefaction by a High Pressure Expansion Process,” the entirety ofwhich is incorporated by reference herein.

This application is related to U.S. Provisional Application No.62/565,733, titled “Natural Gas Liquefaction by a High PressureExpansion Process,” being commonly owned and filed on an even dateherewith, the disclosure of which is incorporated by reference herein inits entirety.

BACKGROUND Field of Disclosure

The disclosure relates generally to liquefied natural gas (LNG)production. More specifically, the disclosure relates to LNG productionat high pressures.

Description of Related Art

This section is intended to introduce various aspects of the art, whichmay be associated with the present disclosure. This discussion isintended to provide a framework to facilitate a better understanding ofparticular aspects of the present disclosure. Accordingly, it should beunderstood that this section should be read in this light, and notnecessarily as an admission of prior art.

Because of its clean burning qualities and convenience, natural gas hasbecome widely used in recent years. Many sources of natural gas arelocated in remote areas, which are great distances from any commercialmarkets for the gas. Sometimes a pipeline is available for transportingproduced natural gas to a commercial market. When pipelinetransportation is not feasible, produced natural gas is often processedinto liquefied natural gas (LNG) for transport to market.

In the design of an LNG plant, one of the most important considerationsis the process for converting the natural gas feed stream into LNG.Currently, the most common liquefaction processes use some form ofrefrigeration system. Although many refrigeration cycles have been usedto liquefy natural gas, the three types most commonly used in LNG plantstoday are: (1) the “cascade cycle,” which uses multiple single componentrefrigerants in heat exchangers arranged progressively to reduce thetemperature of the gas to a liquefaction temperature; (2) the“multi-component refrigeration cycle,” which uses a multi-componentrefrigerant in specially designed exchangers; and (3) the “expandercycle,” which expands gas from feed gas pressure to a low pressure witha corresponding reduction in temperature. Most natural gas liquefactioncycles use variations or combinations of these three basic types.

The refrigerants used in liquefaction processes may comprise a mixtureof components such as methane, ethane, propane, butane, and nitrogen inmulti-component refrigeration cycles. The refrigerants may also be puresubstances such as propane, ethylene, or nitrogen in “cascade cycles.”Substantial volumes of these refrigerants with close control ofcomposition are required. Further, such refrigerants may have to beimported and stored, which impose logistics requirements, especially forLNG production in remote locations. Alternatively, some of thecomponents of the refrigerant may be prepared, typically by adistillation process integrated with the liquefaction process.

The use of gas expanders to provide the feed gas cooling, therebyeliminating or reducing the logistical problems of refrigerant handling,is seen in some instances as having advantages over refrigerant-basedcooling. The expander system operates on the principle that therefrigerant gas can be allowed to expand through an expansion turbine,thereby performing work and reducing the temperature of the gas. The lowtemperature gas is then heat exchanged with the feed gas to provide therefrigeration needed. The power obtained from cooling expansions in gasexpanders can be used to supply part of the main compression power usedin the refrigeration cycle. The typical expander cycle for making LNGoperates at the feed gas pressure, typically under about 6,895 kPa(1,000 psia). Supplemental cooling is typically needed to fully liquefythe feed gas and this may be provided by additional refrigerant systems,such as secondary cooling and/or sub-cooling loops. For example, U.S.Pat. Nos. 6,412,302 and 5,916,260 present expander cycles which describethe use of nitrogen as refrigerant in the sub-cooling loop.

Previously proposed expander cycles have all been less efficientthermodynamically, however, than the current natural gas liquefactioncycles based on refrigerant systems. Expander cycles have therefore notoffered any installed cost advantage to date, and liquefaction cyclesinvolving refrigerants are still the preferred option for natural gasliquefaction.

Because expander cycles result in a high recycle gas stream flow rateand high inefficiency for the primary cooling (warm) stage, gasexpanders have typically been used to further cool feed gas after it hasbeen pre-cooled to temperatures well below −20° C. using an externalrefrigerant in a closed cycle, for example. Thus, a common factor inmost proposed expander cycles is the requirement for a second, externalrefrigeration cycle to pre-cool the gas before the gas enters theexpander. Such a combined external refrigeration cycle and expandercycle is sometimes referred to as a “hybrid cycle.” While suchrefrigerant-based pre-cooling eliminates a major source of inefficiencyin the use of expanders, it significantly reduces the benefits of theexpander cycle, namely the elimination of external refrigerants.

U. S. Patent Application US2009/0217701 introduced the concept of usinghigh pressure within the primary cooling loop to eliminate the need forexternal refrigerant and improve efficiency, at least comparable to thatof refrigerant-based cycles currently in use. The high pressure expanderprocess (HPXP), disclosed in U. S. Patent Application US2009/0217701, isan expander cycle which uses high pressure expanders in a mannerdistinguishing from other expander cycles. A portion of the feed gasstream may be extracted and used as the refrigerant in either an openloop or closed loop refrigeration cycle to cool the feed gas streambelow its critical temperature. Alternatively, a portion of LNG boil-offgas may be extracted and used as the refrigerant in a closed looprefrigeration cycle to cool the feed gas stream below its criticaltemperature. This refrigeration cycle is referred to as the primarycooling loop. The primary cooling loop is followed by a sub-cooling loopwhich acts to further cool the feed gas. Within the primary coolingloop, the refrigerant is compressed to a pressure greater than 1,500psia, or more preferably, to a pressure of approximately 3,000 psia. Therefrigerant is then cooled against an ambient cooling medium (air orwater) prior to being near isentropically expanded to provide the coldrefrigerant needed to liquefy the feed gas.

FIG. 1 depicts an example of a known HPXP liquefaction process 100, andis similar to one or more processes disclosed in U. S. PatentApplication US2009/0217701. In FIG. 1, an expander loop 102 (i.e., anexpander cycle) and a sub-cooling loop 104 are used. Feed gas stream 106enters the HPXP liquefaction process at a pressure less than about 1,200psia, or less than about 1,100 psia, or less than about 1,000 psia, orless than about 900 psia, or less than about 800 psia, or less thanabout 700 psia, or less than about 600 psia. Typically, the pressure offeed gas stream 106 will be about 800 psia. Feed gas stream 106generally comprises natural gas that has been treated to removecontaminants using processes and equipment that are well known in theart.

In the expander loop 102, a compression unit 108 compresses arefrigerant stream 109 (which may be a treated gas stream) to a pressuregreater than or equal to about 1,500 psia, thus providing a compressedrefrigerant stream 110. Alternatively, the refrigerant stream 109 may becompressed to a pressure greater than or equal to about 1,600 psia, orgreater than or equal to about 1,700 psia, or greater than or equal toabout 1,800 psia, or greater than or equal to about 1,900 psia, orgreater than or equal to about 2,000 psia, or greater than or equal toabout 2,500 psia, or greater than or equal to about 3,000 psia, thusproviding compressed refrigerant stream 110. After exiting compressionunit 108, compressed refrigerant stream 110 is passed to a cooler 112where it is cooled by indirect heat exchange with a suitable coolingfluid to provide a compressed, cooled refrigerant stream 114. Cooler 112may be of the type that provides water or air as the cooling fluid,although any type of cooler can be used. The temperature of thecompressed, cooled refrigerant stream 114 depends on the ambientconditions and the cooling medium used, and is typically from about 35°F. to about 105° F. Compressed, cooled refrigerant stream 114 is thenpassed to an expander 116 where it is expanded and consequently cooledto form an expanded refrigerant stream 118. Expander 116 is awork-expansion device, such as a gas expander, which produces work thatmay be extracted and used for compression. Expanded refrigerant stream118 is passed to a first heat exchanger 120, and provides at least partof the refrigeration duty for first heat exchanger 120. Upon exitingfirst heat exchanger 120, expanded refrigerant stream 118 is fed to acompression unit 122 for pressurization to form refrigerant stream 109.

Feed gas stream 106 flows through first heat exchanger 120 where it iscooled, at least in part, by indirect heat exchange with expandedrefrigerant stream 118. After exiting first heat exchanger 120, the feedgas stream 106 is passed to a second heat exchanger 124. The principalfunction of second heat exchanger 124 is to sub-cool the feed gasstream. Thus, in second heat exchanger 124 the feed gas stream 106 issub-cooled by sub-cooling loop 104 (described below) to producesub-cooled stream 126. Sub-cooled stream 126 is then expanded to a lowerpressure in expander 128 to form a liquid fraction and a remaining vaporfraction. Expander 128 may be any pressure reducing device, including,but not limited to a valve, control valve, Joule Thompson valve, Venturidevice, liquid expander, hydraulic turbine, and the like. The sub-cooledstream 126, which is now at a lower pressure and partially liquefied, ispassed to a surge tank 130 where the liquefied fraction 132 is withdrawnfrom the process as an LNG stream 134, which has a temperaturecorresponding to the bubble point pressure. The remaining vapor fraction(flash vapor) stream 136 may be used as fuel to power the compressorunits.

In sub-cooling loop 104, an expanded sub-cooling refrigerant stream 138(preferably comprising nitrogen) is discharged from an expander 140 anddrawn through second and first heat exchangers 124, 120. Expandedsub-cooling refrigerant stream 138 is then sent to a compression unit142 where it is re-compressed to a higher pressure and warmed. Afterexiting compression unit 142, the re-compressed sub-cooling refrigerantstream 144 is cooled in a cooler 146, which can be of the same type ascooler 112, although any type of cooler may be used. After cooling, there-compressed sub-cooling refrigerant stream is passed to first heatexchanger 120 where it is further cooled by indirect heat exchange withexpanded refrigerant stream 118 and expanded sub-cooling refrigerantstream 138. After exiting first heat exchanger 120, the re-compressedand cooled sub-cooling refrigerant stream is expanded through expander140 to provide a cooled stream which is then passed through second heatexchanger 124 to sub-cool the portion of the feed gas stream to befinally expanded to produce LNG.

U. S. Patent Application US2010/0107684 disclosed an improvement to theperformance of the HPXP through the discovery that adding externalcooling to further cool the compressed refrigerant to temperatures belowambient conditions provides significant advantages which in certainsituations justifies the added equipment associated with externalcooling. The HPXP embodiments described in the aforementioned patentapplications perform comparably to alternative mixed externalrefrigerant LNG production processes such as single mixed refrigerantprocesses. However, there remains a need to further improve theefficiency of the HPXP as well as overall train capacity. There remainsa particular need to improve the efficiency of the HPXP in cases wherethe feed gas pressure is less than 1,200 psia.

U. S. Patent Application 2010/0186445 disclosed the incorporation offeed compression up to 4,500 psia to the HPXP. Compressing the feed gasprior to liquefying the gas in the HPXP's primary cooling loop has theadvantage of increasing the overall process efficiency. For a givenproduction rate, this also has the advantage of significantly reducingthe required flow rate of the refrigerant within the primary coolingloop which enables the use of compact equipment, which is particularlyattractive for floating LNG applications. Furthermore, feed compressionprovides a means of increasing the LNG production of an HPXP train bymore than 30% for a fixed amount of power going to the primary coolingand sub-cooling loops. This flexibility in production rate is againparticularly attractive for floating LNG applications where there aremore restrictions than land based applications in matching the choice ofrefrigerant loop drivers with desired production rates. Althoughliquefying the feed gas at high pressures has advantages, it was foundthat for liquefaction pressures greater than 1,500 psia the choice ofsuitable cryogenic heat exchangers for the primary cooling andsub-cooling loops were limited to options significantly high in cost,weight and with reduced fluid processing capabilities. For example, theuse of printed circuit heat exchangers, which are capable of operatingat pressures greater than 4,500 psia, was shown to significantlyincrease project cost compared to the more widely sourced brazedaluminum heat exchanger type where proven operating pressures are lessthan 1,500 psia. This significant increase in cost may limit thepractical application of feed compression to up to 1,500 psia. Thus,there remains a need to further improve the HPXP without requiring feedcompression or feed compression greater the 1,500 psia. Additionally,there remains an additional need to allow the use of significant feedcompression with HPXP without requiring the use of high-cost maincryogenic heat exchangers such as printed circuit heat exchangers.

SUMMARY

The present disclosure provides a method for liquefying a feed gasstream rich in methane using a system having first and second heatexchanger zones, where the method comprises the following steps:providing the feed gas stream at a pressure less than 1,200 psia;providing a compressed refrigerant stream with a pressure greater thanor equal to 1,500 psia; cooling the compressed refrigerant stream byindirect heat exchange with an ambient temperature air or water, toproduce a compressed, cooled refrigerant stream; directing thecompressed, cooled refrigerant stream to the second heat exchanger zoneto additionally cool the compressed, cooled refrigerant stream belowambient temperature to produce a compressed, additionally cooledrefrigerant stream; expanding the compressed, additionally cooledrefrigerant stream in at least one work producing expander, therebyproducing an expanded, cooled refrigerant stream; passing the expanded,cooled refrigerant stream through the first heat exchanger zone to forma first warm refrigerant stream, wherein the first warm refrigerantstream has a temperature that is cooler, by at least 5° F., than thehighest fluid temperature within the first heat exchanger zone; passingthe feed gas stream through the first heat exchanger zone to cool atleast part of the feed gas stream by indirect heat exchange with theexpanded, cooled refrigerant stream, thereby forming a liquefied gasstream; directing at least a portion of the first warm refrigerantstream to the second heat exchanger zone to cool by indirect heatexchange the compressed, cooled refrigerant stream, thereby forming asecond warm refrigerant stream; and compressing the second warmrefrigerant stream to produce the compressed refrigerant stream.

The present disclosure also provides a system for liquefying a feed gasstream rich in methane, the system having first and second heatexchanger zones. A feed gas stream at a pressure less than 1,200 psia isprovided. A compressed refrigerant stream is provided with a pressuregreater than or equal to 1,500 psia. A cooler is configured to cool thecompressed refrigerant stream by indirect heat exchange with an ambienttemperature air or water, to produce a compressed, cooled refrigerantstream. At least one heat exchanger is provided within the second heatexchanger zone, the compressed, cooled refrigerant stream being directedto the at least one heat exchanger within the second heat exchanger zoneto additionally cool the compressed, cooled refrigerant stream belowambient temperature and thereby produce a compressed, additionallycooled refrigerant stream. At least one work producing expander isarranged to expand the compressed, additionally cooled refrigerantstream, thereby producing an expanded, cooled refrigerant stream. Atleast one heat exchanger is provided within the first heat exchangerzone. The expanded, cooled refrigerant stream passes through the atleast one heat exchanger in the first heat exchanger zone to form afirst warm refrigerant stream, wherein the first warm refrigerant streamhas a temperature that is cooler, by at least 5° F., than the highestfluid temperature within the first heat exchanger zone. The feed gasstream passes through the first heat exchanger zone to cool at leastpart of the feed gas stream by indirect heat exchange with the expanded,cooled refrigerant stream, thereby forming a liquefied gas stream. Atleast a portion of the first warm refrigerant stream is directed to thesecond heat exchanger zone to cool by indirect heat exchange thecompressed, cooled refrigerant stream, thereby forming a second warmrefrigerant stream. A compressor is configured to compress the secondwarm refrigerant stream to produce the compressed refrigerant stream.

The present disclosure also provides a method for liquefying a feed gasstream rich in methane, where the method comprises the following steps:providing the feed gas stream at a pressure less than 1,200 psia;compressing the feed gas stream to a pressure of at least 1,500 psia toform a compressed gas stream; cooling the compressed gas stream byindirect heat exchange with an ambient temperature air or water, to forma cooled, compressed gas stream; expanding the cooled, compressed gasstream in at least one work producing expander to a pressure that isless than 2,000 psia and no greater than the pressure to which the gasstream was compressed, to thereby form a chilled gas stream; providing acompressed refrigerant stream with a pressure greater than or equal to1,500 psia; cooling the compressed refrigerant stream by indirect heatexchange with an ambient temperature air or water, to produce acompressed, cooled refrigerant stream; directing the compressed, cooledrefrigerant stream to a second heat exchanger zone, to additionally coolthe compressed, cooled refrigerant stream below ambient temperature, toproduce a compressed, additionally cooled refrigerant stream; expandingthe compressed, additionally cooled refrigerant stream in at least onework producing expander, thereby producing an expanded, cooledrefrigerant stream; passing the expanded, cooled refrigerant streamthrough a first heat exchanger zone to form a first warm refrigerantstream, whereby the first warm refrigerant stream has a temperature thatis cooler, by at least 5° F., than the highest fluid temperature withinthe first heat exchanger zone; passing the chilled gas stream throughthe first heat exchanger zone to cool at least part of the chilled gasstream by indirect heat exchange with the expanded, cooled refrigerant,thereby forming a liquefied gas stream; directing the first warmrefrigerant stream to the second heat exchanger zone to cool by indirectheat exchange the compressed, cooled refrigerant stream, thereby forminga second warm refrigerant stream; and compressing the second warmrefrigerant stream to produce the compressed refrigerant stream.

The disclosure also provides a system for liquefying a feed gas streamrich in methane and having a pressure less than 1,200 psia. The systemcomprises: a compressor for compressing the feed gas stream to apressure of at least 1,500 psia, to form a compressed gas stream; acooler for cooling the compressed gas stream by indirect heat exchangewith an ambient temperature air or water, to form a cooled, compressedgas stream; at least one work producing expander for expanding thecooled, compressed gas stream to a pressure that is less than 2,000 psiaand no greater than the pressure to which the gas stream was compressed,to thereby form a chilled gas stream; a compressed refrigerant streamwith a pressure greater than or equal to 1,500 psia; a refrigerantcooler for cooling the compressed refrigerant stream by indirect heatexchange with an ambient temperature air or water, to produce acompressed, cooled refrigerant stream; a heat exchanger zone throughwhich the compressed, cooled refrigerant stream is directed to beadditionally cooled below ambient temperature, to produce a compressed,additionally cooled refrigerant stream; an additional work producingexpander for expanding the compressed, additionally cooled refrigerantstream, thereby producing an expanded, cooled refrigerant stream; anadditional heat exchanger zone through which the expanded, cooledrefrigerant stream is passed, to thereby form a first warm refrigerantstream, whereby the warm refrigerant stream has a temperature that iscooler, by at least 5° F., than the highest fluid temperature within thefirst heat exchanger zone; wherein the chilled gas stream is passedthrough the additional heat exchanger zone to cool at least part of thechilled gas stream by indirect heat exchange with the expanded, cooledrefrigerant, thereby forming a liquefied gas stream; wherein the firstwarm refrigerant stream is directed to the heat exchanger zone to coolby indirect heat exchange the compressed, cooled refrigerant stream,thereby forming a second warm refrigerant stream; and an additionalcompressor for compressing the second warm refrigerant stream to producethe compressed refrigerant stream.

The foregoing has broadly outlined the features of the presentdisclosure so that the detailed description that follows may be betterunderstood. Additional features will also be described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the disclosure willbecome apparent from the following description, appending claims and theaccompanying drawings, which are briefly described below.

FIG. 1 is a schematic diagram of a system for LNG production accordingto known principles.

FIG. 2 is a schematic diagram of a system for LNG production accordingto disclosed aspects.

FIG. 3 is a schematic diagram of a system for LNG production accordingto disclosed aspects.

FIG. 4 is a schematic diagram of a system for LNG production accordingto disclosed aspects.

FIG. 5 is a schematic diagram of a system for LNG production accordingto disclosed aspects.

FIG. 6 is a schematic diagram of a system for LNG production accordingto disclosed aspects.

FIG. 7 is a schematic diagram of a system for LNG production accordingto disclosed aspects.

FIG. 8 is a schematic diagram of a system for LNG production accordingto disclosed aspects.

FIG. 9 is a schematic diagram of a system for LNG production accordingto disclosed aspects.

FIG. 10 is a flowchart of a method according to aspects of thedisclosure.

FIG. 11 is a flowchart of a method according to aspects of thedisclosure.

FIG. 12 is a flowchart of a method according to aspects of thedisclosure.

It should be noted that the figures are merely examples and nolimitations on the scope of the present disclosure are intended thereby.Further, the figures are generally not drawn to scale, but are draftedfor purposes of convenience and clarity in illustrating various aspectsof the disclosure.

DETAILED DESCRIPTION

To promote an understanding of the principles of the disclosure,reference will now be made to the features illustrated in the drawingsand specific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of thedisclosure is thereby intended. Any alterations and furthermodifications, and any further applications of the principles of thedisclosure as described herein are contemplated as would normally occurto one skilled in the art to which the disclosure relates. For the sakeof clarity, some features not relevant to the present disclosure may notbe shown in the drawings.

At the outset, for ease of reference, certain terms used in thisapplication and their meanings as used in this context are set forth. Tothe extent a term used herein is not defined below, it should be giventhe broadest definition persons in the pertinent art have given thatterm as reflected in at least one printed publication or issued patent.Further, the present techniques are not limited by the usage of theterms shown below, as all equivalents, synonyms, new developments, andterms or techniques that serve the same or a similar purpose areconsidered to be within the scope of the present claims.

As one of ordinary skill would appreciate, different persons may referto the same feature or component by different names. This document doesnot intend to distinguish between components or features that differ inname only. The figures are not necessarily to scale. Certain featuresand components herein may be shown exaggerated in scale or in schematicform and some details of conventional elements may not be shown in theinterest of clarity and conciseness. When referring to the figuresdescribed herein, the same reference numerals may be referenced inmultiple figures for the sake of simplicity. In the followingdescription and in the claims, the terms “including” and “comprising”are used in an open-ended fashion, and thus, should be interpreted tomean “including, but not limited to.”

The articles “the,” “a” and “an” are not necessarily limited to meanonly one, but rather are inclusive and open ended so as to include,optionally, multiple such elements.

As used herein, the terms “approximately,” “about,” “substantially,” andsimilar terms are intended to have a broad meaning in harmony with thecommon and accepted usage by those of ordinary skill in the art to whichthe subject matter of this disclosure pertains. It should be understoodby those of skill in the art who review this disclosure that these termsare intended to allow a description of certain features described andclaimed without restricting the scope of these features to the precisenumeral ranges provided. Accordingly, these terms should be interpretedas indicating that insubstantial or inconsequential modifications oralterations of the subject matter described and are considered to bewithin the scope of the disclosure. The term “near” is intended to meanwithin 2%, or within 5%, or within 10%, of a number or amount.

As used herein, the term “compression unit” means any one type orcombination of similar or different types of compression equipment, andmay include auxiliary equipment, known in the art for compressing asubstance or mixture of substances. A “compression unit” may utilize oneor more compression stages. Illustrative compressors may include, butare not limited to, positive displacement types, such as reciprocatingand rotary compressors for example, and dynamic types, such ascentrifugal and axial flow compressors, for example.

“Exemplary” is used exclusively herein to mean “serving as an example,instance, or illustration.” Any embodiment or aspect described herein as“exemplary” is not to be construed as preferred or advantageous overother embodiments.

The term “gas” is used interchangeably with “vapor,” and is defined as asubstance or mixture of substances in the gaseous state as distinguishedfrom the liquid or solid state. Likewise, the term “liquid” means asubstance or mixture of substances in the liquid state as distinguishedfrom the gas or solid state.

As used herein, “heat exchange area” means any one type or combinationof similar or different types of equipment known in the art forfacilitating heat transfer. Thus, a “heat exchange area” may becontained within a single piece of equipment, or it may comprise areascontained in a plurality of equipment pieces. Conversely, multiple heatexchange areas may be contained in a single piece of equipment.

A “hydrocarbon” is an organic compound that primarily includes theelements hydrogen and carbon, although nitrogen, sulfur, oxygen, metals,or any number of other elements can be present in small amounts. As usedherein, hydrocarbons generally refer to components found in natural gas,oil, or chemical processing facilities.

As used herein, the terms “loop” and “cycle” are used interchangeably.

As used herein, “natural gas” means a gaseous feedstock suitable formanufacturing LNG, where the feedstock is a methane-rich gas containingmethane (C1) as a major component. Natural gas may include gas obtainedfrom a crude oil well (associated gas) or from a gas well(non-associated gas).

The disclosure describes a process/method and system for liquefyingnatural gas and other methane-rich gas streams to produce liquefiednatural gas (LNG) and/or other liquefied methane-rich gases. In one ormore aspects of the disclosure, the primary cooling loop is segmentedinto two heat exchanger zones. Within the first heat exchanger zone, theprimary cooling loop refrigerant is used to liquefy the feed gas. Withinthe second heat exchanger zone, all or a portion of the primary coolingloop refrigerant is used to cool the high pressure primary cooling looprefrigerant prior to expansion of the refrigerant. The first heatexchanger zone is physically separate from second heat exchanger zone.Additionally, the heat exchanger type of the first heat exchanger zoneis different from the heat exchanger type of the second heat exchangerzone. One advantage of having two separate heat exchanger zones is thatthe types of heat exchangers in the two zones can be different from eachother. As a non-limiting example, the type of heat exchanger(s) used inthe first exchanger zone may include a brazed aluminum heat exchanger,and the type of heat exchanger(s) used in the second heat exchanger zonemay be include a printed circuit heat exchanger. It is in the firstexchanger zone where more the 90% of the heat transfer needed to liquefythe feed gas occurs. Using the less expensive brazed aluminum heatexchanger here reduces project cost. The significantly more expensiveprinted circuit heat exchanger may be used in the second heat exchangerzone because it can operate at the required 3,000 psia pressure of thehigh pressure refrigerant. The use of a printed circuit heat exchangerin the second heat exchanger zone does not significantly impact overallproject cost since it is a relatively small heat exchanger. This isbecause the heat transfer duty within the second heat exchanger zone issignificantly smaller than that of the first heat exchanger zone. Bothheat exchanger zones may comprise multiple heat exchangers.

In an aspect, a method for liquefying a gas stream, particularly onerich in methane, includes: (a) providing the gas stream at a pressureless than 1,200 psia; (b) providing a compressed refrigerant with apressure greater than or equal to 1,500 psia; (c) cooling the compressedrefrigerant by indirect heat exchange with an ambient temperature air orwater to produce a compressed, cooled refrigerant; (d) directing thecompressed, cooled refrigerant to a second heat exchanger zone toadditionally cool the compressed, cooled refrigerant below ambienttemperature to produce a compressed, additionally cooled refrigerant;(e) expanding the compressed, additionally cooled refrigerant in atleast one work producing expander thereby producing an expanded, cooledrefrigerant; (f) passing the expanded, cooled refrigerant through afirst heat exchanger zone to form a first warm refrigerant, whereby thefirst warm refrigerant has a temperature that is cooler, by at least 5°F., than the highest fluid temperature within the first heat exchangerzone, and whereby the heat exchanger type of the first heat exchangerzone is different from the heat exchanger type of the second heatexchanger zone; (g) passing the gas stream through the first heatexchanger zone to cool at least part of the gas stream by indirect heatexchange with the expanded, cooled refrigerant, thereby forming aliquefied gas stream; (h) directing at least a portion of the first warmrefrigerant to the second heat exchanger zone to cool by indirect heatexchange the compressed, cooled refrigerant thereby forming a secondwarm refrigerant; and (i) compressing the second warm refrigerant toproduce the compressed refrigerant.

In another aspect, a method for liquefying a gas stream includes: (a)providing the gas stream at a pressure less than 1,200 psia; (b)compressing the gas stream to a pressure of at least 1,500 psia to forma compressed gas stream; (c) cooling the compressed gas stream byindirect heat exchange with an ambient temperature air or water to forma compressed, cooled gas stream; (d) expanding the compressed, cooledgas stream in at least one work producing expander to a pressure that isless than 2,000 psia and no greater than the pressure to which the gasstream was compressed, to thereby form a chilled gas stream; (e)providing a compressed refrigerant with a pressure greater than or equalto 1,500 psia (f) cooling the compressed refrigerant by indirect heatexchange with an ambient temperature air or water to produce acompressed, cooled refrigerant (g) directing the compressed, cooledrefrigerant to a second heat exchanger zone to additionally cool thecompressed, cooled refrigerant below ambient temperature to produce acompressed, additionally cooled refrigerant; (h) expanding thecompressed, additionally cooled refrigerant in at least one workproducing expander thereby producing an expanded, cooled refrigerant;(i) passing the expanded, cooled refrigerant through a first heatexchanger zone to form a first warm refrigerant, whereby the first warmrefrigerant has a temperature that is cooler, by at least 5° F., thanthe highest fluid temperature within the first heat exchanger zone, andwhereby the heat exchanger type of the first heat exchanger zone isdifferent from the heat exchanger type of the second heat exchangerzone; (j) passing the chilled gas stream through the first heatexchanger zone to cool at least part of the chilled gas stream byindirect heat exchange with the expanded, cooled refrigerant, therebyforming a liquefied gas stream; (k) directing the first warm refrigerantto the second heat exchanger zone to cool by indirect heat exchange thecompressed, cooled refrigerant, thereby forming a second warmrefrigerant; and (l) compressing the second warm refrigerant to producethe compressed refrigerant.

In another aspect, a method for liquefying a gas stream includes: (a)providing the gas stream at a pressure less than 1,200 psia; (b)providing a refrigerant stream at near the same pressure of the gasstream; (c) mixing the gas stream with the refrigerant stream to form asecond gas stream; (d) compressing the second gas stream to a pressureof at least 1,500 psia to form a compressed second gas stream; (e)cooling the compressed second gas stream by indirect heat exchange withan ambient temperature air or water to form a compressed, cooled secondgas stream; (f) directing the compressed, cooled second gas stream to asecond heat exchanger zone to additionally cool the compressed, cooledsecond gas stream below ambient temperature to produce a compressed,additionally cooled second gas stream; (g) expanding the compressed,additionally cooled second gas stream in at least one work producingexpander to a pressure that is less than 2,000 psia and no greater thanthe pressure to which the second gas stream was compressed, to therebyform an expanded, cooled second gas stream; (h) separating the expanded,cooled second gas stream into a first expanded refrigerant and a chilledgas stream; (i) expanding the first expanded refrigerant in at least onework producing expander, thereby producing a second expandedrefrigerant; (j) passing the second expanded refrigerant through a firstheat exchanger zone to form a first warm refrigerant, whereby the firstwarm refrigerant has a temperature that is cooler, by at least 5° F.,than the highest fluid temperature within the first heat exchanger zone,and whereby the heat exchanger type of the first heat exchanger zone isdifferent from the heat exchanger type of the second heat exchangerzone; (k) passing the chilled gas stream through the first heatexchanger zone to cool at least part of the chilled gas stream byindirect heat exchange with the second expanded refrigerant, therebyforming a liquefied gas stream; (l) directing the first warm refrigerantto the second heat exchanger zone to cool by indirect heat exchange thecompressed, cooled second gas stream, thereby forming a second warmrefrigerant; and (m) compressing the second warm refrigerant to producethe refrigerant stream.

Aspects of the disclosure may include the additional steps ofcompressing the gas stream to a pressure no greater than 1,600 psia andthen cooling the compressed gas stream by indirect heat exchange with anambient temperature air or water prior to directing the gas stream tothe first heat exchanger zone. Aspects of the disclosure may alsoinclude the additional steps of cooling the gas stream to a temperaturebelow the ambient by indirect heat exchange within an external coolingunit prior to directing the gas stream to the first heat exchanger zone.Aspects of the disclosure may also include the additional steps ofcooling the compressed, cooled refrigerant to a temperature below theambient temperature by indirect heat exchange with an external coolingunit prior to directing the compressed, cooled refrigerant to the secondheat exchanger zone. These described additional steps may be employedsingularly or in combination with each other.

Aspects of the disclosure have several advantages over the knownliquefaction processes, in which feed compression is required tosignificantly improve the efficiency of the HPXP. In contrast, theefficiency of the disclosed aspects is more than 16% greater than theefficiency for a comparable configuration according to knownliquefaction processes. Aspects of the disclosure may have theadditional advantage of allowing significant feed compression (greaterthan 1,500 psia) without requiring the use of high cost main cryogenicheat exchangers for the first heat exchanger zone. Feed compression bythe disclosed method may provide a means of increasing the LNGproduction of an HPXP train by more than 25% for a fixed amount of powergoing to the primary cooling and sub-cooling loops. Aspects of thedisclosure may also have the advantage of combining the compressionservice of the feed gas and some of that of the primary cooling loop toreduce equipment count. Such an embodiment provides a highly efficientand compact configuration suitable for small scale LNG applications.

FIG. 2 is a schematic diagram that illustrates a liquefaction system 200according to an aspect of the disclosure. The liquefaction system 200includes a primary cooling loop 202, which may also be called anexpander loop. The liquefaction system also includes a sub-cooling loop204, which is a closed refrigeration loop preferably charged withnitrogen as the sub-cooling refrigerant. Within the primary cooling loop202, an expanded, cooled refrigerant stream 205 is directed to a firstheat exchanger zone 201 where it exchanges heat with a feed gas stream206 to form a first warm refrigerant stream 208. A portion of the firstwarm refrigerant 208 is directed to a second heat exchanger zone 210where, in one or more heat exchangers 210 a, it exchanges heat with acompressed, cooled refrigerant stream 212 to additionally cool thecompressed, cooled refrigerant stream and form a second warm refrigerantstream 209 and a compressed, additionally cooled refrigerant stream 213.The one or more heat exchangers 210 a may be of a printed circuit heatexchanger type, a shell and tube heat exchanger type, or a combinationthereof. The heat exchanger types within the second heat exchanger zonemay have a design pressure of greater than 1,500 psia, or morepreferably, a design pressure of greater than 2,000 psia, or morepreferably, a design pressure of greater than 3,000 psia.

The portion of the first warm refrigerant stream 208 directed to thesecond heat exchanger zone 210 has a temperature that is cooler by atleast 5° F., or more preferably, cooler by at least 10° F., or morepreferably, cooler by at least 15° F., than the highest fluidtemperature within the first heat exchanger zone 201. The portion of thefirst warm refrigerant stream 208 that may remain within the first heatexchanger zone (as shown by reference number 208 a) further exchangesheat with the feed gas stream to form a third warm refrigerant stream214. The second warm refrigerant stream 209 from the second heatexchanger zone 210 may be combined with the third warm refrigerantstream 214 from the first heat exchanger zone 201 to produce a fourthwarm refrigerant stream 216. The fourth warm refrigerant stream iscompressed in one or more compression units 218, 220 to a pressuregreater than 1,500 psia, or more preferably, to a pressure ofapproximately 3,000 psia, to form a compressed refrigerant stream 222.The compressed refrigerant stream 222 is then cooled against an ambientcooling medium (air or water) in a cooler 224 to produce the compressed,cooled refrigerant stream 212. Cooler 224 may be similar to cooler 112as previously described. The compressed, additionally cooled refrigerantstream 213 is near isentropically expanded in an expander 226 to producethe expanded, cooled refrigerant stream 205. Expander 226 may be awork-expansion device, such as a gas expander, which produces work thatmay be extracted and used for compression.

The first heat exchanger zone 201 may include a plurality of heatexchanger devices, and in the aspects shown in FIG. 2, the first heatexchanger zone includes first and second main heat exchangers 232, 234,and a sub-cooling heat exchanger 236 exchange heat with the expanded,cooled refrigerant 205. These heat exchangers may be of a brazedaluminum heat exchanger type, a plate fin heat exchanger type, a spiralwound heat exchanger type, or a combination thereof. Within thesub-cooling loop 204, an expanded sub-cooling refrigerant stream 238(preferably comprising nitrogen) is discharged from an expander 240 anddrawn through sub-cooling heat exchanger 236 and second and first mainheat exchangers 234, 232. Expanded sub-cooling refrigerant stream 238 isthen sent to a compression unit 242 where it is re-compressed to ahigher pressure and warmed. After exiting compression unit 242, there-compressed sub-cooling refrigerant stream 244 is cooled in a cooler246, which can be of the same type as cooler 224, although any type ofcooler may be used. After cooling, the re-compressed sub-coolingrefrigerant stream is passed through first and second main heatexchangers 232, 234 where it is further cooled by indirect heat exchangewith part or all of the warm refrigerant stream 208 and expandedsub-cooling refrigerant stream 238. After exiting first heat exchangearea 201, the re-compressed and cooled sub-cooling refrigerant stream isexpanded through expander 240 to provide the expanded sub-cooledrefrigerant stream 238 that is re-cycled through the first heatexchanger zone as described herein. In this manner, the feed gas stream206 is cooled, liquefied and sub-cooled in the first heat exchanger zone201 to produce a sub-cooled gas stream 248. Sub-cooled gas stream 248 isthen expanded to a lower pressure in expander 250 to form a liquidfraction and a remaining vapor fraction. Expander 250 may be anypressure reducing device, including but not limited to a valve, controlvalve, Joule Thompson valve, Venturi device, liquid expander, hydraulicturbine, and the like. The sub-cooled stream 248, which is now at alower pressure and partially liquefied, is passed to a surge tank 252where the liquefied fraction 254 is withdrawn from the process as an LNGstream 256, which has a temperature corresponding to the bubble pointpressure. The remaining vapor fraction (flash vapor) stream 258 may beused as fuel to power the compressor units.

FIG. 3 is a schematic diagram that illustrates a liquefaction system 300according to another aspect of the disclosure. Liquefaction system 300is similar to liquefaction system 200 and for the sake of brevitysimilarly depicted or numbered components may not be further described.Liquefaction system 300 includes a primary cooling loop 302 and asub-cooling loop 304. Liquefaction system 300 also includes first andsecond heat exchanger zones 301, 310. In contrast with liquefactionsystem 200, all of the first warm refrigerant 308 is directed to thesecond heat exchanger zone 310 where, in one or more heat exchangers 310a, it exchanges heat with a compressed, cooled refrigerant stream 312 toform a second warm refrigerant 309.

The first warm refrigerant stream 308 has a temperature that is coolerby at least 5° F., or more preferably, cooler by at least 10° F., ormore preferably, cooler by at least 15° F., than the highest fluidtemperature within the first heat exchanger zone. The second warmrefrigerant stream 309 may be compressed in one or more compressors 318,320 to a pressure greater than 1,500 psia, or more preferably, to apressure of approximately 3,000 psia, to thereby form a compressedrefrigerant stream 322. The compressed refrigerant stream 322 is thencooled against an ambient cooling medium (air or water) to produce thecompressed, cooled refrigerant stream 312 that is directed to the secondheat exchanger zone 310. The compressed, additionally cooled refrigerantstream 313 is near isentropically expanded in an expander 326 to producethe expanded, cooled refrigerant stream 305.

The feed gas stream 306 is directed through the first heat exchange area301 that includes a main heat exchanger 332 and a sub-cooling heatexchanger 336. The number of main heat exchangers in first heatexchanger zone 301 may be reduced since all of the first warmrefrigerant 308 is directed to the second heat exchanger zone 310.Within the sub-cooling loop 304, an expanded sub-cooling refrigerantstream 338 (preferably comprising nitrogen) is discharged from anexpander 340 and drawn through sub-cooling heat exchanger 336 and mainheat exchanger 332. Expanded sub-cooling refrigerant stream 338 is thensent to a compression unit 342 where it is re-compressed to a higherpressure and warmed. After exiting compression unit 342, there-compressed sub-cooling refrigerant stream 344 is cooled in a cooler346, which can be of the same type as cooler 324, although any type ofcooler may be used. After cooling, the re-compressed sub-coolingrefrigerant stream is passed through main heat exchanger 232 where it isfurther cooled by indirect heat exchange with part or all of theexpanded, cooled refrigerant stream 305 and expanded sub-coolingrefrigerant stream 338. After exiting first heat exchange area 301, there-compressed and cooled sub-cooling refrigerant stream is expandedthrough expander 340 to provide the expanded sub-cooled refrigerantstream 338 that is re-cycled through the first heat exchange area asdescribed herein. In this manner, the feed gas stream 306 is cooled,liquefied and sub-cooled in the first heat exchanger zone 301 to producea sub-cooled gas stream 348. Sub-cooled gas stream 348 is then expandedto a lower pressure in expander 350 to form a liquid fraction and aremaining vapor fraction. Expander 350 may be any pressure reducingdevice, including but not limited to a valve, control valve, JouleThompson valve, Venturi device, liquid expander, hydraulic turbine, andthe like. The sub-cooled stream 348, which is now at a lower pressureand partially liquefied, is passed to a surge tank 352 where theliquefied fraction 354 is withdrawn from the process as an LNG stream356, which has a temperature corresponding to the bubble point pressure.The remaining vapor fraction (flash vapor) stream 358 may be used asfuel to power the compressor units.

FIG. 4 is a schematic diagram that illustrates a liquefaction system 400according to another aspect of the disclosure. Liquefaction system 400is similar to liquefaction system 200, and for the sake of brevitysimilarly depicted or numbered components may not be further described.Liquefaction system 400 includes a primary cooling loop 402 and asub-cooling loop 404. Liquefaction system 400 also includes first andsecond heat exchanger zones 401, 410. In liquefaction system 400, thesub-cooling loop 404 is an open refrigeration loop where a portion 449of the expanded, sub-cooled gas stream 448 is recycled and used as thesub-cooling refrigerant stream. Specifically, the portion 449 of theexpanded, sub-cooled gas stream is directed through the first heatexchanger zone 401 as previously described before being compressed in acompressor 442, cooled in a cooler 446, and re-inserted into the feedgas stream 406. This sub-cooling refrigerant stream may be one stream,as shown, or may comprise multiple streams at different pressures: forexample, a portion of the expanded, sub-cooling gas stream—not to exceed50% thereof—may be diverted and pass through one or more pressurereduction valves to reduce its pressure to a range of about 30 to 300psia, to thereby produce one or more reduced pressure gas streams. Thereduced pressure gas streams may then be passed through the first heatexchanger zone as the sub-cooling refrigerant. Having multiple streamsimproves the efficiency of the sub-cooling process. Alternatively, thissub-cooling loop may be configured to be a closed refrigeration loop.

FIG. 5 is a schematic diagram that illustrates a liquefaction system 500according to another aspect of the disclosure. Liquefaction system 500is similar to liquefaction system 200 and for the sake of brevitysimilarly depicted or numbered components may not be further described.Liquefaction system 500 includes a primary cooling loop 502 and asub-cooling loop 504. Liquefaction system 500 also includes first andsecond heat exchanger zones 501, 510. Liquefaction system 500 streamincludes the additional steps of compressing the feed gas stream 506 ina compressor 560 and then, using a cooler 562, cooling the compressedfeed gas 561 with ambient air or water to produce a cooled, compressedfeed gas stream 563. Feed gas compression may be used to improve theoverall efficiency of the liquefaction process and increase LNGproduction.

FIG. 6 is a schematic diagram that illustrates a liquefaction system 600according to still another aspect of the disclosure. Liquefaction system600 is similar to liquefaction system 300 and for the sake of brevitysimilarly depicted or numbered components may not be further described.Liquefaction system 600 includes a primary cooling loop 602 and asub-cooling loop 604. Liquefaction system 600 also includes first andsecond heat exchanger zones 601, 610. Liquefaction system 600 includesthe additional step of chilling, in an external cooling unit 665, thefeed gas stream 606 to a temperature below the ambient temperature toproduce a chilled gas stream 667. The chilled gas stream 667 is thendirected to the first heat exchanger zone 601 as previously described.Chilling the feed gas as shown in FIG. 6 may be used to improve theoverall efficiency of the liquefaction process and increase LNGproduction.

FIG. 7 is a schematic diagram that illustrates a liquefaction system 700according to another aspect of the disclosure. Liquefaction system 700is similar to liquefaction system 200 and for the sake of brevitysimilarly depicted or numbered components may not be further described.Liquefaction system 700 includes a primary cooling loop 702 and asub-cooling loop 704. Liquefaction system 700 also includes first andsecond heat exchanger zones 701, 710. Liquefaction system 700 includesthe additional step of chilling, using an external cooling unit 770, thecompressed, cooled refrigerant 712 in the primary cooling loop 702 to atemperature below the ambient temperature, to thereby produce acompressed, chilled refrigerant 772. The compressed, chilled refrigerant772 is then directed to the second heat exchanger zone 710 as previouslydescribed. Using an external cooling unit to further cool thecompressed, cool refrigerant may be used to improve the overallefficiency of the process and increase LNG production.

FIG. 8 is a schematic diagram that illustrates a liquefaction system 800according to another aspect of the disclosure. Liquefaction system 800is similar to liquefaction system 300 and for the sake of brevitysimilarly depicted or numbered components may not be further described.Liquefaction system 800 includes a primary cooling loop 802 and asub-cooling loop 804. Liquefaction system 800 also includes first andsecond heat exchanger zones 801, 810. In liquefaction system 800, thefeed gas stream 806 is compressed in a compressor 860 to a pressure ofat least 1,500 psia to form a compressed gas stream 861. Using anexternal cooling unit 862, the compressed gas stream 861 is cooled byindirect heat exchange with an ambient temperature air or water to forma compressed, cooled gas stream 863. The compressed, cooled gas stream863 is expanded in at least one work producing expander 874 to apressure that is less than 2,000 psia but no greater than the pressureto which the gas stream was compressed, to thereby form a chilled gasstream 876. The chilled gas stream 876 is then directed to the firstheat exchanger zone 801 where a primary cooling refrigerant and asub-cooling refrigerant are used to liquefy the chilled gas stream aspreviously described.

The sub-cooling loop 804 is a closed refrigeration loop preferablycharged with nitrogen as the sub-cooling refrigerant stream. Within theprimary cooling loop 802, an expanded, cooled refrigerant stream 805 isdirected to the first heat exchanger zone 801 where it exchanges heatwith the chilled gas stream 876 to form a first warm refrigerant stream808. The first warm refrigerant stream 808 is directed to the secondheat exchanger zone 810 where it exchanges heat with a compressed,cooled refrigerant stream 825 to additionally cool the compressed,cooled refrigerant stream 825, thereby forming a second warm refrigerantstream 809 and a compressed, additionally cooled refrigerant stream 813.The first warm refrigerant stream 808 has a temperature that is coolerby at least 5° F., or more preferably, cooler by at least 10° F., ormore preferably, cooler by at least 15° F., than the highest fluidtemperature within the first heat exchanger zone 801. Using one or morecompressors 818, 820, the second warm refrigerant stream 809 iscompressed to a pressure greater than 1,500 psia, or more preferably, toa pressure of approximately 3,000 psia, to form a compressed refrigerantstream 822. The compressed refrigerant stream 822 is then cooled againstan ambient cooling medium (air or water) in an external cooling unit 824to produce the compressed, cooled refrigerant stream 825. After beingdirected through the second heat exchanger area 810, the compressed,additionally cooled refrigerant stream is near isentropically expandedin an expander 826 to produce the expanded, cooled refrigerant 805. Thechilled gas stream 876 is liquefied and sub-cooled in the first heatexchanger zone to produce a sub-cooled gas stream 848, which is furtherprocessed as previously disclosed.

FIG. 9 is a schematic diagram that illustrates a liquefaction system 900according to yet another aspect of the disclosure. Liquefaction system900 contains similar structure and components with previously disclosedliquefaction systems and for the sake of brevity similarly depicted ornumbered components may not be further described. Liquefaction system900 includes a primary cooling loop 902 and a sub-cooling loop 904.Liquefaction system 900 also includes first and second heat exchangerzones 901, 910. In liquefaction system 900, the feed gas stream 906 ismixed with a refrigerant stream 907 to produce a second feed gas stream906 a. Using a compressor 960, the second feed gas stream 906 a iscompressed to a pressure greater than 1,500 psia, or more preferably, toa pressure of approximately 3,000 psia, to form a compressed second gasstream 961. Using an external cooling unit 962, the compressed secondgas stream 961 is then cooled against an ambient cooling medium (air orwater) to produce a compressed, cooled second gas stream 963. Thecompressed, cooled second gas stream 963 is directed to the second heatexchanger zone 910 where it exchanges heat with a first warm refrigerantstream 908, to produce a compressed, additionally cooled second gasstream 913 and a second warm refrigerant stream 909.

The compressed, additionally cooled second gas stream 913 is expanded inat least one work producing expander 926 to a pressure that is less than2,000 psia, but no greater than the pressure to which the second gasstream 906 a was compressed, to thereby form an expanded, cooled secondgas stream 980. The expanded, cooled second gas stream 980 is separatedinto a first expanded refrigerant stream 905 and a chilled feed gasstream 906 b. The first expanded refrigerant stream 905 may be nearisentropically expanded using an expander 982 to form a second expandedrefrigerant stream 905 a. The chilled feed gas stream 906 b is directedto the first heat exchanger zone 901 where a primary cooling refrigerant(i.e., the second expanded refrigerant stream 905 a) and a sub-coolingrefrigerant (from the sub-cooling loop 904) are used to liquefy thechilled gas stream 906 b. The sub-cooling loop 904 may be a closedrefrigeration loop, preferably charged with nitrogen as the sub-coolingrefrigerant. Within the primary cooling loop 902, the second expandedrefrigerant stream 905 a is directed to the first heat exchanger zone901 where it exchanges heat with the chilled feed gas stream 906 b toform the first warm refrigerant stream 908. The first warm refrigerantstream 908 may have a temperature that is cooler by at least 5° F., ormore preferably, cooler by at least 10° F., or more preferably, coolerby at least 15° F., than the highest fluid temperature within the firstheat exchanger zone 901. The second warm refrigerant stream 909 iscompressed in one or more compressors 918 and then cooled with anambient cooling medium in an external cooling device 924 to produce therefrigerant stream 907. The chilled feed gas stream 906 b is liquefiedand sub-cooled in the first heat exchanger zone 901 to produce asub-cooled gas stream 948, which is processed as previously described toform LNG.

Aspects of the disclosure illustrated in FIG. 9 demonstrate that theprimary refrigerant stream may comprise part of the feed gas stream,which in a preferred aspect may be primarily or nearly all methane.Indeed, it may be advantageous for the refrigerant in the primarycooling loop of all the disclosed aspects (i.e., FIGS. 2 through 9) becomprised of at least 85% methane, or at least 90% methane, or at least95% methane, or greater than 95% methane. This is because methane may bereadily available in various parts of the disclosed processes, and theuse of methane may eliminate the need to transport refrigerants toremote LNG processing locations. As a non-limiting example, therefrigerant in the primary cooling loop 202 in FIG. 2 may be takenthrough line 206 a of the feed gas stream 206 if the feed gas is highenough in methane to meet the compositions as described above.Alternatively, part or all of a boil-off gas stream 259 from an LNGstorage tank 257 may be used to supply refrigerant for the primarycooling loop 202. Furthermore, if the feed gas stream is sufficientlylow in nitrogen, part or all of the end flash gas stream 258 (whichwould then be low in nitrogen) may be used to supply refrigerant for theprimary cooling loop 202. Lastly, any combination of line 206 a,boil-off gas stream 259, and end flash gas stream 258 may be used toprovide or even occasionally replenish the refrigerant in the primarycooling loop 202.

FIG. 10 is a flowchart of a method 1000 for liquefying a feed gas streamrich in methane using a system having first and second heat exchangerzones, where the method comprises the following steps: 1002, providingthe feed gas stream at a pressure less than 1,200 psia; 1004, providinga compressed refrigerant stream with a pressure greater than or equal to1,500 psia; 1006, cooling the compressed refrigerant stream by indirectheat exchange with an ambient temperature air or water, to produce acompressed, cooled refrigerant stream; 1008, directing the compressed,cooled refrigerant stream to the second heat exchanger zone toadditionally cool the compressed, cooled refrigerant stream belowambient temperature to produce a compressed, additionally cooledrefrigerant stream; 1010, expanding the compressed, additionally cooledrefrigerant stream in at least one work producing expander, therebyproducing an expanded, cooled refrigerant stream; 1012, passing theexpanded, cooled refrigerant stream through the first heat exchangerzone to form a first warm refrigerant stream, wherein the first warmrefrigerant stream has a temperature that is cooler, by at least 5° F.,than the highest fluid temperature within the first heat exchanger zone;1014, passing the feed gas stream through the first heat exchanger zoneto cool at least part of the feed gas stream by indirect heat exchangewith the expanded, cooled refrigerant stream, thereby forming aliquefied gas stream; 1016 directing at least a portion of the firstwarm refrigerant stream to the second heat exchanger zone to cool byindirect heat exchange the compressed, cooled refrigerant stream,thereby forming a second warm refrigerant stream; and 1018, compressingthe second warm refrigerant stream to produce the compressed refrigerantstream.

FIG. 11 is a flowchart of a method 1100 for liquefying a feed gas streamrich in methane, where the method comprises the following steps: 1102,providing the feed gas stream at a pressure less than 1,200 psia; 1104,compressing the feed gas stream to a pressure of at least 1,500 psia toform a compressed gas stream; 1106, cooling the compressed gas stream byindirect heat exchange with an ambient temperature air or water, to forma cooled, compressed gas stream; 1108, expanding the cooled, compressedgas stream in at least one work producing expander to a pressure that isless than 2,000 psia and no greater than the pressure to which the gasstream was compressed, to thereby form a chilled gas stream; 1110,providing a compressed refrigerant stream with a pressure greater thanor equal to 1,500 psia; 1112, cooling the compressed refrigerant streamby indirect heat exchange with an ambient temperature air or water, toproduce a compressed, cooled refrigerant stream; 1114, directing thecompressed, cooled refrigerant stream to a second heat exchanger zone,to additionally cool the compressed, cooled refrigerant stream belowambient temperature, to produce a compressed, additionally cooledrefrigerant stream; 1116, expanding the compressed, additionally cooledrefrigerant stream in at least one work producing expander, therebyproducing an expanded, cooled refrigerant stream; 1118, passing theexpanded, cooled refrigerant stream through a first heat exchanger zoneto form a first warm refrigerant stream, whereby the first warmrefrigerant stream has a temperature that is cooler, by at least 5° F.,than the highest fluid temperature within the first heat exchanger zone;1120, passing the chilled gas stream through the first heat exchangerzone to cool at least part of the chilled gas stream by indirect heatexchange with the expanded, cooled refrigerant, thereby forming aliquefied gas stream; 1122, directing the first warm refrigerant streamto the second heat exchanger zone to cool by indirect heat exchange thecompressed, cooled refrigerant stream, thereby forming a second warmrefrigerant stream;

and 1124, compressing the second warm refrigerant stream to produce thecompressed refrigerant stream.

FIG. 12 is a method 1200 for liquefying a feed gas stream rich inmethane, where the method comprises the following steps: 1202, providingthe feed gas stream at a pressure less than 1,200 psia; 1204, providinga refrigerant stream at near the same pressure of the feed gas stream;1206, mixing the feed gas stream with the refrigerant stream to form asecond gas stream; 1208, compressing the second gas stream to a pressureof at least 1,500 psia to form a compressed second gas stream; 1210,cooling the compressed second gas stream by indirect heat exchange withambient temperature air or water, to form a compressed, cooled secondgas stream; 1212, directing the compressed, cooled second gas stream toa second heat exchanger zone, to additionally cool the compressed,cooled second gas stream below ambient temperature, thereby producing acompressed, additionally cooled second gas stream; 1214, expanding thecompressed, additionally cooled second gas stream in at least one workproducing expander to a pressure that is less than 2,000 psia and nogreater than the pressure to which the second gas stream was compressed,to thereby form an expanded, cooled second gas stream; 1216, separatingthe expanded, cooled second gas stream into a first expanded refrigerantstream and a chilled gas stream; 1218, expanding the first expandedrefrigerant stream in at least one work producing expander, therebyproducing a second expanded refrigerant stream; 1220, passing the secondexpanded refrigerant stream through a first heat exchanger zone to forma first warm refrigerant stream such that the first warm refrigerantstream has a temperature that is cooler, by at least 5° F., than thehighest fluid temperature within the first heat exchanger zone; 1222,passing the chilled gas stream through the first heat exchanger zone tocool at least part of the chilled gas stream by indirect heat exchangewith the second expanded refrigerant stream, thereby forming a liquefiedgas stream; 1224, directing the first warm refrigerant stream to thesecond heat exchanger zone to cool by indirect heat exchange thecompressed, cooled second gas stream, thereby forming a second warmrefrigerant stream; and 1226, compressing the second warm refrigerantstream to produce the refrigerant stream.

The steps depicted in FIGS. 10-12 are provided for illustrative purposesonly and a particular step may not be required to perform the disclosedmethodology. Moreover, FIGS. 10-12 may not illustrate all the steps thatmay be performed. The claims, and only the claims, define the disclosedsystem and methodology.

The aspects described herein have several advantages over knowntechnologies. For example, the described technology may greatly reducethe size and cost of systems that treat sour natural gas.

It should be understood that the numerous changes, modifications, andalternatives to the preceding disclosure can be made without departingfrom the scope of the disclosure. The preceding description, therefore,is not meant to limit the scope of the disclosure. Rather, the scope ofthe disclosure is to be determined only by the appended claims and theirequivalents. It is also contemplated that structures and features in thepresent examples can be altered, rearranged, substituted, deleted,duplicated, combined, or added to each other.

What is claimed is:
 1. A method for liquefying a feed gas stream rich inmethane using a system having first and second heat exchanger zones,where the method comprises: (a) providing the feed gas stream at apressure less than 1,200 psia; (b) providing a compressed refrigerantstream with a pressure greater than or equal to 1,500 psia; (c) coolingthe compressed refrigerant stream by indirect heat exchange with anambient temperature air or water, to produce a compressed, cooledrefrigerant stream; (d) directing the compressed, cooled refrigerantstream to the second heat exchanger zone to additionally cool thecompressed, cooled refrigerant stream below ambient temperature toproduce a compressed, additionally cooled refrigerant stream; (e)expanding the compressed, additionally cooled refrigerant stream in atleast one work producing expander, thereby producing an expanded, cooledrefrigerant stream; (f) passing the expanded, cooled refrigerant streamthrough the first heat exchanger zone to form a first warm refrigerantstream, wherein the first warm refrigerant stream has a temperature thatis cooler, by at least 5° F., than the highest fluid temperature withinthe first heat exchanger zone; (g) passing the feed gas stream throughthe first heat exchanger zone to cool at least part of the feed gasstream by indirect heat exchange with the expanded, cooled refrigerantstream, thereby forming a liquefied gas stream; (h) directing at least aportion of the first warm refrigerant stream to the second heatexchanger zone to cool by indirect heat exchange the compressed, cooledrefrigerant stream, thereby forming a second warm refrigerant stream;and (i) compressing the second warm refrigerant stream to produce thecompressed refrigerant stream.
 2. The method of claim 1, wherein thefirst warm refrigerant stream has a temperature that is cooler, by atleast 10° F., than the highest fluid temperature within the first heatexchanger zone.
 3. The method of claim 1, wherein a portion of the firstwarm refrigerant stream remaining within the first heat exchanger zonefurther exchanges heat within the first heat exchanger zone to produce athird warm refrigerant stream.
 4. The method of claim 1, furthercomprising: combining the second warm refrigerant stream with the thirdwarm refrigerant stream prior to compressing the second warm refrigerantstream.
 5. The method of claim 1, further comprising: further coolingthe liquefied gas stream within the first heat exchanger zone using asub-cooling refrigeration cycle, to thereby form a sub-cooled gasstream.
 6. The method of claim 1, further comprising: expanding thesub-cooled gas stream in a hydraulic turbine to a pressure greater thanor equal to 50 psia and less than or equal to 450 psia, to produce anexpanded, sub-cooled gas stream.
 7. The method of claim 5, wherein thesub-cooling refrigeration cycle comprises a closed loop gas phaserefrigeration cycle using nitrogen gas as a refrigerant.
 8. The methodof claim 6, wherein the sub-cooling refrigeration cycle comprises:withdrawing a portion not to exceed 50% of the expanded, sub-cooled gasstream and reducing its pressure in a pressure reduction valve to arange of about 30 to 300 psia to produce one or more reduced pressuregas streams; and passing the one or more reduced pressure gas streamsthrough the first heat exchanger zone as the sub-cooling refrigerant. 9.The method of claim 8, wherein the one or more reduced pressure gasstreams comprise two or more reduced pressure gas streams havingdifferent pressures from each other.
 10. The method of claim 8, furthercomprising: compressing the sub-cooling refrigerant stream exiting thefirst heat exchanger zone; and cooling the sub-cooling refrigerantstream by indirect heat exchange with an ambient temperature air orwater and then adding the sub-cooling refrigerant to the gas stream. 11.The method of claim 6, wherein at least a portion of the expanded,sub-cooled gas stream is further expanded and then directed to aseparation tank from which liquid natural gas is withdrawn and remaininggaseous vapors are withdrawn as a flash gas stream.
 12. The method ofclaim 1, wherein all of the first warm refrigerant stream is directed tothe second heat exchanger zone to cool by indirect heat exchange thecompressed, cooled refrigerant stream, thereby forming the second warmrefrigerant stream.
 13. The method of claim 1, further comprising: priorto directing the feed gas stream to the first heat exchanger zone,compressing the feed gas stream to a pressure no greater 1,600 psia, andthen cooling it by indirect heat exchange with an ambient temperatureair or water.
 14. The method of claim 1, wherein the feed gas stream iscooled to a temperature below an ambient temperature by indirect heatexchange within an external cooling unit prior to directing the feed gasstream to the first heat exchanger zone.
 15. The method of claim 1,wherein the compressed, cooled refrigerant stream is cooled to atemperature below the ambient temperature by indirect heat exchangewithin an external cooling unit prior to directing the compressed,cooled refrigerant stream to the second heat exchanger zone.
 16. Asystem for liquefying a feed gas stream rich in methane, the systemhaving first and second heat exchanger zones and comprising: a feed gasstream at a pressure less than 1,200 psia; a compressed refrigerantstream with a pressure greater than or equal to 1,500 psia; a coolerconfigured to cool the compressed refrigerant stream by indirect heatexchange with an ambient temperature air or water, to produce acompressed, cooled refrigerant stream; at least one heat exchangerwithin the second heat exchanger zone, the compressed, cooledrefrigerant stream being directed to the at least one heat exchangerwithin the second heat exchanger zone to additionally cool thecompressed, cooled refrigerant stream below ambient temperature andthereby produce a compressed, additionally cooled refrigerant stream; atleast one work producing expander arranged to expand the compressed,additionally cooled refrigerant stream, thereby producing an expanded,cooled refrigerant stream; at least one heat exchanger within the firstheat exchanger zone, the expanded, cooled refrigerant stream beingpassed through the at least one heat exchanger in the first heatexchanger zone to form a first warm refrigerant stream, wherein thefirst warm refrigerant stream has a temperature that is cooler, by atleast 5° F., than the highest fluid temperature within the first heatexchanger zone; wherein the feed gas stream is passed through the firstheat exchanger zone to cool at least part of the feed gas stream byindirect heat exchange with the expanded, cooled refrigerant stream,thereby forming a liquefied gas stream; wherein at least a portion ofthe first warm refrigerant stream is directed to the second heatexchanger zone to cool by indirect heat exchange the compressed, cooledrefrigerant stream, thereby forming a second warm refrigerant stream;and a compressor configured to compress the second warm refrigerantstream to produce the compressed refrigerant stream.
 17. The system ofclaim 16, wherein the first warm refrigerant stream has a temperaturethat is cooler, by at least 10° F., than the highest fluid temperaturewithin the first heat exchanger zone.
 18. The system of claim 16,wherein a portion of the first warm refrigerant stream remaining withinthe first heat exchanger zone further exchanges heat within the firstheat exchanger zone to produce a third warm refrigerant stream.
 19. Thesystem of claim 16, wherein the second warm refrigerant stream iscombined with the third warm refrigerant stream prior to compressing thesecond warm refrigerant stream.
 20. The system of claim 16, furthercomprising: a sub-cooling refrigeration cycle configured to further coolthe liquefied gas stream within the first heat exchanger zone, tothereby form a sub-cooled gas stream.
 21. The system of claim 16,further comprising: an additional expander configured to expand thesub-cooled gas stream to a pressure greater than or equal to 50 psia andless than or equal to 450 psia, to produce an expanded, sub-cooled gasstream, wherein the additional expander comprises a hydraulic turbine.22. The system of claim 20, wherein the sub-cooling refrigeration cyclecomprises a closed loop gas phase refrigeration cycle using nitrogen gasas a refrigerant.
 23. The system of claim 21, further comprising: apressure reduction valve configured to reduce the pressure of a portion,not to exceed 50%, of the expanded, sub-cooled gas stream, to a range ofabout 30 to 300 psia, thereby producing one or more reduced pressure gasstreams; wherein the one or more reduced pressure gas streams is passedthrough the first heat exchanger zone as the sub-cooling refrigerant.24. The system of claim 23, wherein the one or more reduced pressure gasstreams comprise two or more reduced pressure gas streams havingdifferent pressures from each other.
 25. The system of claim 20, furthercomprising: a sub-cooling compressor configured to compress thesub-cooling refrigerant stream exiting the first heat exchanger zone;and an external cooling unit configured to cool the sub-coolingrefrigerant stream by indirect heat exchange with an ambient temperatureair or water.
 26. The system of claim 21, further comprising; anadditional expander configured to further expand at least a portion ofthe expanded, sub-cooled gas stream; and a separation tank to which theexpanded, sub-cooled gas stream is directed after passing through theadditional expander.
 27. The system of claim 16, further comprising: anadditional compressor configured to compress, prior to directing thefeed gas stream to the first heat exchanger zone, the feed gas stream toa pressure no greater 1,600 psia; and an external cooling unitconfigured to cool the feed gas stream by indirect heat exchange with anambient temperature air or water.
 28. The system of claim 16, furthercomprising: a second external cooling unit configured to cool the feedgas stream to a temperature below an ambient temperature by indirectheat exchange within an external cooling unit prior to directing thefeed gas stream to the first heat exchanger zone.
 29. The system ofclaim 16, further comprising: a third external cooling unit configuredto cool the compressed, cooled refrigerant stream to a temperature belowthe ambient temperature by indirect heat exchange therein prior todirecting the compressed, cooled refrigerant stream to the second heatexchanger zone.
 30. The system of claim 26, wherein refrigerant in theprimary cooling loop is supplied from one or more of the feed gasstream, the flash gas stream, and boil-off gas of the liquid naturalgas.
 31. The system of claim 16, wherein at least one heat exchangerwithin the first heat exchanger zone comprises a brazed aluminum heatexchanger.
 32. The system of claim 16, wherein at least one heatexchanger within the second heat exchanger zone comprises a printedcircuit heat exchanger.